JP2006176674A - Cold heat transportation medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cold heat transportation medium containing tetra-n-butylammonium bromide hydrate having low corrosiveness, particularly low local corrosiveness. <P>SOLUTION: The cold heat transportation medium is a tetra-n-butylammonium bromide aqueous solution containing at least one corrosionproof agent selected from a sulfite and a thiosulfate, and an alkali hydroxide. The cold and heat transportation medium contains 30 to 20,000 wt. ppm above corrosionproof agent in terms of a sulfite ion. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a cold transport medium used in a heat storage system such as an air conditioner, and more specifically to a tetra n-butylammonium bromide hydrate cold transport medium with suppressed corrosivity.

  Conventionally, various heat storage devices are used in, for example, air conditioning equipment. The heat storage device is used, for example, to store energy with discontinuous supply such as late-night power or factory waste heat, and effectively uses energy by using the stored cold energy in an air conditioning facility.

As such a heat storage device, one using ice is known. A heat storage device that uses ice produces ice at night using late-night power or the like, and uses the cold energy stored in the ice in daytime air conditioning equipment. This heat storage device using ice has an advantage that a larger amount of cold energy can be stored by the latent heat of ice compared to a heat storage device using sensible heat of water.
However, ice requires cooling water to a temperature well below its freezing point in order to produce it, while the freezing point of water is low, which reduces the coefficient of performance of the refrigerator. In addition, since ice is difficult to handle and transport, there is a problem that the apparatus becomes complicated and large.

On the other hand, a heat storage device using a hydrate is also known. Hydrates can be generated at temperatures above the freezing point of water by selecting the type of guest molecule and other conditions, and can form a slurry of the hydrate particles in water. As a heat storage medium composed of such a hydrate, a gas hydrate which is a kind of clathrate hydrate is disclosed. (For example, see Patent Document 1).
However, the technique described in this publication uses a fluorocarbon refrigerant R11 as a refrigerant for generating a gas hydrate. Since the fluorocarbon refrigerant R11 is a substance having a large ozone depletion coefficient and is a gas under atmospheric pressure, it needs to use a sealed container, and there is a problem that the heat storage device becomes expensive.

  In the course of conducting research to provide a hydrate-based cold transport medium that can be produced at a temperature higher than ice without using a special refrigerant, tetra- We focused on butylammonium hydrate. The hydrate of tetra n-butylammonium bromide obtained by cooling an aqueous solution of tetra n-butylammonium bromide is cooled by ordinary cooling water, brine water or the like without using a special refrigerant. It can be easily obtained as a slurry and has a large amount of retained heat.

Although this tetra n-butylammonium bromide can be used after cooling, it can also be used at a high temperature for heating.
However, since tetra n-butylammonium bromide contains corrosive components such as bromine which is a halogen ion in the solution, there is a risk of corroding metal piping and equipment constituting the circulation system. In particular, when used at high temperatures, corrosion is severe and difficult to adapt with current piping materials.

Therefore, pipes have been conventionally taken to cope with corrosion, and as an example, an aqueous solution of tetra n-butylammonium bromide was selected from the group consisting of sodium nitrite, sodium sulfite, sodium pyrophosphate and benzotriazole. A technique for adding a corrosion inhibitor is known (see Patent Document 2).
JP-A-2-203138 JP 2001-172617 A

  However, when the anticorrosion effect of the technology of Patent Document 2 on the metal material that is subject to corrosion prevention was examined, it was effective in preventing overall corrosion. When the agent is added alone, it has been found that pitting corrosion such as stainless steel, and local corrosion such as crevice corrosion of iron and iron-based alloys, copper and copper-based alloys may be caused.

  The present invention has been made to solve such a problem, and an object of the present invention is to provide a cold transport medium containing tetra n-butylammonium bromide hydrate having reduced corrosivity, particularly local corrosivity. To do.

(1) The cold transport medium according to the present invention is an aqueous solution of tetra n-butylammonium bromide and contains at least one corrosion inhibitor selected from sulfites and thiosulfates and an alkali hydroxide. It is characterized by doing.
As the sulfite, sodium sulfite, lithium sulfite and calcium sulfite are preferable, and as the thiosulfate, sodium thiosulfate, lithium thiosulfate and calcium thiosulfate are preferable. As the alkali hydroxide, calcium hydroxide, sodium hydroxide, and lithium hydroxide are preferable.

(2) Further, it is an aqueous solution of tetra n-butylammonium bromide, containing at least one corrosion inhibitor selected from sulfites and thiosulfates, and the pH of the aqueous solution is 8-10. It is characterized by this.

(3) Moreover, the corrosion inhibitor described in the above (1) or (2) is contained at a concentration of 30 ppm to 20000 ppm in terms of sulfite ion.

(4) Moreover, the alkali hydroxide described in the above (1) is contained at a concentration of 200 ppm by weight or less.

(5) In the above (1), the corrosion inhibitor is contained in a concentration of 30 to 20000 ppm by weight in terms of sulfite ion, and the alkali hydroxide is contained in a concentration of 200 ppm by weight or less. It is characterized by being.

  According to the present invention, a cold transport medium comprising tetra n-butylammonium bromide hydrate with reduced corrosivity, particularly local corrosivity, is obtained.

  A slurry of tetra n-butylammonium bromide (hereinafter referred to as “TBAB”) hydrate can be produced by cooling an aqueous solution of TBAB. The concentration of the TBAB aqueous solution is not particularly limited, but it is usually preferable to use an aqueous solution containing TBAB at a concentration of 5 to 42% by weight. Then, the TBAB aqueous solution may be cooled from 12 ° C. within a temperature range of 5 to 12 ° C. used in a general heat storage air conditioning system to a temperature lower than that, preferably 8 ° C. to 5 ° C. desirable.

The cold heat transport medium containing the TBAB hydrate slurry of the present invention contains at least one corrosion inhibitor selected from sulfites and thiosulfates as a corrosion inhibitor, and further contains an alkali hydroxide. By containing these corrosion inhibitors and alkali hydroxides, corrosion by TBAB hydrate slurry of the metal piping material and the equipment material constituting the circulation system of the cold transport medium can be suppressed or reduced. In particular, it has a great effect of suppressing local corrosion such as pitting corrosion, crevice corrosion of stainless steel, crevice corrosion of iron, iron-based alloys, copper, and copper-based alloys, and airflow difference corrosion.
Hereinafter, the mechanism of suppressing and reducing the corrosion will be described. First, the effects of sulfite and thiosulfate will be described.

The corrosive environment of the cold transport medium including the TBAB hydrate slurry is created by halogen ions contained in the cold transport medium and dissolved oxygen contained in the cold transport medium aqueous solution.
However, since the pH of the TBAB hydrate slurry is in a neutral range, the cathode reaction of dissolved oxygen rather than halogen ions is the main driving reaction for corrosion.
Therefore, the system that circulates the cold heat transport medium is basically a closed system, and the dissolved oxygen in the medium is reduced as much as possible by performing gas replacement degassing, hollow fiber vacuum degassing, and the like.

  However, although it is a closed system, air may enter from the valve or pump in the circulation system. In addition, when the expansion tank, the heat storage tank, or the like is an open system, or when a new medium is added when the cold transport medium leaks or decreases, the air enters the circulation system. In this way, even if the dissolved oxygen in the medium is reduced as much as possible, there is actually intrusion of air, which causes troubles.

In this respect, the corrosion inhibitor used in the conventional circulating water piping has a small inhibitory force against corrosion against these intruded dissolved oxygen in the TBAB hydrate slurry environment. In addition, a corrosion inhibitor that promotes passivation, such as nitrite, can suppress overall corrosion relatively, but has little inhibitory power against local corrosion such as pitting corrosion, differential airflow corrosion, and galvanic corrosion.
In addition, these corrosion inhibitors usually have a critical concentration, and if less than a certain concentration, corrosion may be promoted or local corrosion (pitting corrosion or crevice corrosion) may occur compared to no addition. is there.

On the other hand, the cold transport medium of the present invention contains sulfite or thiosulfate as a corrosion inhibitor, so that sulfite or thiosulfate is self-oxidized with respect to dissolved oxygen entering the circulation system. Dissolved oxygen can be consumed and dissolved oxygen contributing to corrosion can be reduced. Therefore, it is possible to suppress the occurrence of corrosion that could not be suppressed with conventional corrosion inhibitors.
In addition, a continuous effect is expectable, so that there is much content of sulfite or thiosulfate.
However, at the room temperature, the lower limit is 30 ppm by weight in terms of sulfite ion, which is necessary for autooxidation in order to change the cold transport medium from the saturated dissolved oxygen state to the completely degassed state. It is preferable that the upper limit is 20000 ppm by weight, which is the maximum dissolution amount in.
The upper limit was specified because excessive content may cause precipitation at the time of use (7 ° C), resulting in clogging of the pipe, or formation of a film on the metal surface, which may cause crevice corrosion. is there.

The cold transport medium containing TBAB hydrate of the present invention contains an alkali hydroxide in addition to at least one corrosion inhibitor selected from the group consisting of sulfites and thiosulfates as a corrosion inhibitor. Below, the effect of containing an alkali hydroxide is demonstrated.
By containing at least one type of corrosion inhibitor selected from sulfite and thiosulfate in the TBAB aqueous solution, the dissolved oxygen in the aqueous solution can be reduced as described above, and overall corrosion can be suppressed. Furthermore, local corrosion such as crevice corrosion and pitting corrosion can be suppressed by adding alkali hydroxide to bring the pH of the TBAB aqueous solution to an appropriate range.
Moreover, in the case where copper or a copper-based alloy is used together with iron or an iron-based alloy as a metal material constituting the circulation system of the cold heat transport medium by containing an alkali hydroxide, the copper in the TBAB hydrate slurry It is possible to suppress and reduce ion elution and to suppress corrosion of iron and iron-based alloys.
Hereinafter, the mechanism of this corrosion suppression will be described.

In copper and copper alloys, the corrosion resistance in a high-temperature TBAB aqueous solution may be somewhat lowered by sulfate ions or ammonia that may be generated in the circulation system, and elution of copper ions occurs.
The dissolved copper ions become a substance that exhibits a cathode reaction with respect to iron or iron-based alloy in the circulation system, and promotes corrosion.
Therefore, in order to suppress corrosion, it is necessary to suppress and reduce elution of copper ions.
In this respect, elution of copper ions of copper and copper-based alloys can be reduced by shifting the pH of the aqueous solution to weak alkaline, and the range of pH 8 to 10 has the greatest effect of suppressing elution. However, the pH of an aqueous solution in which TBAB is dissolved is usually about 7 to 8, which is not sufficient for suppressing copper ion elution.
Therefore, in the present invention, the pH of the aqueous solution of the cold transport medium containing TBAB is adjusted to 8 to 10 by containing alkali hydroxide.
As a result, elution of copper and a copper-based alloy can be suppressed, and as a result, corrosion promotion due to copper ions of iron and an iron-based alloy can be reduced.
In addition, since self-solubility of copper and a copper-type alloy will become high when pH becomes 11 or more, it is preferable that the content of alkali hydroxide sets the upper limit to 200 weight ppm which is the addition amount from which pH is set to 10.
As the alkali hydroxide, calcium hydroxide, sodium hydroxide, or lithium hydroxide is preferably used.
Thus, by setting the pH of the TBAB aqueous solution to 8 to 10, it is possible to suppress the above-described local corrosion and to suppress the corrosion of circulating iron and iron-based alloys in which copper and copper-based alloy materials are used. The pH can be adjusted to 8 to 10 by adding an alkali hydroxide, but may be adjusted by adding another alkali material.

FIG. 1 is a block diagram showing an example of a heat storage system using the cold transport medium of the present invention. The heat storage system shown in FIG. 1 includes a refrigerator 11, a heat exchanger 12 for producing TBAB hydrate, and a heat storage tank 13. Between the refrigerator 11 and the heat exchanger 12, the line L1 for supplying the water cooled by the refrigerator 11 to the heat exchanger 12, and the heat-exchanged water from the heat exchanger 12 are supplied to the refrigerator 11 A line L2 is provided to circulate to the center. A pump P1 for water conveyance is provided in the middle of the line L1.
The heat exchanger 12 and the heat storage tank 13 are connected by a line L3.
The heat storage tank 13 and the heat exchanger 12 are connected by a line L4, and a circulation pump P2 is provided in the middle thereof.

  In operation, water cooled to, for example, 4 ° C. by the refrigerator 11 is circulated through the heat exchanger 12. At the same time, the TBAB aqueous solution AS containing the corrosion inhibitor of the present invention accommodated in the heat storage tank 13 is sequentially circulated to the heat exchanger 12 and the heat storage tank 13 through the line 4 and the line L3 by driving the circulation pump P2 ( 1st circulation system). In the heat exchanger 12, the TBAB aqueous solution AS is cooled by cooled water to form a hydrate to form a slurry having a large amount of stored cold heat, and is stored in the heat storage tank 13. The heat-exchanged water is circulated to the refrigerator 11 and cooled as described above. The slurry generated from the TBAB aqueous solution stored in the heat storage tank 13 is supplied to the air conditioning equipment to transport cold heat.

  The cold transport medium of the present invention can be produced simply by cooling a TBAB aqueous solution containing a predetermined corrosion inhibitor to a temperature of about 5 ° C. to 8 ° C., and it is not necessary to use a special refrigerant. The used water and brine water can be cooled and used. Moreover, since the corrosiveness of the cold transport medium of the present invention is suppressed, the pipe is hardly corroded.

  Hereinafter, the present invention will be described by way of examples, but the present invention is not limited thereto.

Experimental example 1
TBAB was dissolved in water to a concentration of 25% by weight, and 1000 ppm of sodium sulfite and 100 ppm of calcium hydroxide were added to this aqueous TBAB solution (Examples 1 and 2). After immersing a carbon steel sample or a copper sample in the obtained solution at 35 ° C. or 50 ° C. for 15 days, the sample was taken out, washed with water, dried and weighed. And based on this measured value, the annual corrosion rate of the general corrosion of each sample in one year was computed.
As comparative examples, examples in which 1000 ppm of sodium sulfite was added without adding calcium hydroxide (Comparative Examples 1 and 4), TBAB aqueous solution alone (Comparative Examples 2 and 5), and TBAB aqueous solution alone were degassed by N ₂ gas replacement A similar experiment was performed for (Comparative Examples 3 and 6).
About this experiment, the result of carbon steel is shown in Table 1, and the result of copper is shown in Table 2.

These results show that the addition of sulfite and calcium hydroxide can significantly reduce the corrosion rate for carbon steel and copper compared to the degassing process, and the corrosion rate is higher than when only sulfite is added. It can be further reduced, and it was confirmed that it has an excellent anti-corrosion effect.
Similar results were obtained when sodium thiosulfate was added instead of sulfite.

Experimental example 2
TBAB is dissolved in water to a concentration of 25% by weight, 2000 ppm of sodium sulfite and 100 ppm of calcium hydroxide are added to this TBAB aqueous solution, and the local corrosion occurrence potential and natural potential of stainless steel in a flowing water environment at 50 ° C. Measured with a stat.
Moreover, it measured similarly about the case where only 2000 ppm of sodium sulfite was added to TBAB aqueous solution and TBAB aqueous solution.
These results are shown in Table 3.

  When 2000 ppm of sodium sulfite and 100 ppm of calcium hydroxide are added to the TBAB aqueous solution at 50 ° C, the natural potential of stainless steel is lower than the local corrosion occurrence potential, and the potential difference is 500 mV, when TBAB alone or only sodium sulfite is added. It was shown that it was sufficiently large compared to the above, and it was confirmed that the occurrence of local corrosion could be suppressed.

Experimental example 3
TBAB is dissolved in water to a concentration of 25% by weight, 2000 ppm of sodium sulfite and 100 ppm of calcium hydroxide are added to this TBAB aqueous solution, and stainless steel and copper test plate materials in flowing water at 30, 40, 50 and 60 ° C. A test piece with an artificially prepared gap structure was immersed for 30 days after the PVC plate was brought into contact with the plate as a backing plate, and the occurrence of crevice corrosion and pitting corrosion of each test plate material after the test was observed.
This experiment reveals the critical temperature for crevice corrosion and pitting corrosion.
A similar experiment was performed by adding only 2000 ppm by weight of sodium sulfite.
About this experiment, the result of stainless steel is shown in Table 4, and the result of copper is shown in Table 5.

  From this result, when only sodium sulfite is added, crevice corrosion and pitting corrosion occur at 50 ° C or higher. However, by adding sodium sulfite and calcium hydroxide, crevice corrosion and pores of stainless steel and copper reach a high temperature range. It was confirmed that the occurrence of food could be suppressed.

From the above experimental results, it was demonstrated that according to the present invention, a cold transport medium of an aqueous TBAB solution having reduced corrosivity, particularly local corrosivity, is provided.

It is a block diagram which shows an example of the thermal storage system using the cold transport medium which concerns on one embodiment of this invention.

Explanation of symbols

  11 refrigerator, 12 heat exchanger, 13 heat storage tank, L1-L4 line, P1, P2 pump, AS TBAB aqueous solution.

Claims (5)

A cold transport medium characterized in that it is an aqueous solution of tetra n-butylammonium bromide and contains at least one corrosion inhibitor selected from sulfites and thiosulfates and an alkali hydroxide. An aqueous solution of tetra n-butylammonium bromide, containing at least one corrosion inhibitor selected from sulfites and thiosulfates, and having an aqueous solution pH of 8 to 10. Cold transport medium. The cold transport medium according to claim 1 or 2, wherein the corrosion inhibitor is contained at a concentration of 30 to 20000 ppm by weight in terms of sulfite ion. The cold transport medium according to claim 1, wherein the alkali hydroxide is contained at a concentration of 200 ppm by weight or less. 2. The cooling heat according to claim 1, wherein the corrosion inhibitor is contained at a concentration of 30 to 20000 ppm by weight in terms of sulfite ion, and the alkali hydroxide is contained at a concentration of 200 ppm by weight or less. Transport medium.

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